The present invention relates to an evaporative emission control system for an internal combustion engine, and more particularly to such a system that the emission of evaporative fuel is prevented by maintaining the pressure in a fuel tank at a negative pressure.
For example, Japanese Patent Laid-open No. 11-50919 discloses an evaporative emission control system including an evaporative fuel passage for connecting a fuel tank directly to an intake pipe of an internal combustion engine to maintain the pressure in the fuel tank at a negative pressure (a pressure lower than the atmospheric pressure). This conventional system further includes a tank pressure control valve provided in the evaporative fuel passage. When the pressure in the fuel tank is higher than a target pressure set according to the fuel temperature in the fuel tank, valve opening control of the tank pressure control valve is carried out to maintain the pressure in the fuel tank at the target pressure.
In the conventional system mentioned above, however, the target pressure is set according to the fuel temperature, and even when the fuel temperature is relatively high, the pressure in the fuel tank is maintained at the target pressure over a long period of time. Accordingly, volatile components contained in the fuel decreases, so that the proportion of hard-to-atomize components contained in the fuel to be injected from a fuel injection valve becomes large. As a result, an actual air-fuel ratio is deviated from a desired value to cause a deterioration in combustion condition of the internal combustion engine. Further, when the fuel temperature becomes high, vacuum boiling tends to occur in a fuel pump for pumping up the fuel from the fuel tank, so that the fuel cannot be smoothly supplied.
It is accordingly an object of the present invention to provide an evaporative emission control system which can prevent changes in fuel component in the fuel tank and vacuum boiling in the fuel pump to thereby accurately control the air-fuel ratio to a desired value and ensure smooth supply of the fuel.
In accordance with the present invention, there is provided in an evaporative emission control system of an internal combustion engine, including an evaporative fuel passage for connecting a fuel tank and an intake system for said internal combustion engine, a control valve provided in said evaporative fuel passage for opening and closing said evaporative fuel passage, and control means for controlling the opening degree of said control valve so that the pressure in said fuel tank becomes lower than an atmospheric pressure; the improvement comprising fuel temperature detecting means for detecting the temperature of fuel in said fuel tank; and disabling means for disabling the opening operation of said control valve in the case that said fuel temperature is higher than a predetermined temperature.
The xe2x80x9cpredetermined temperaturexe2x80x9d mentioned above is set as a lowermost temperature at which vacuum boiling of the fuel tends to occur in a fuel pump, or as a temperature at which the distillation of the fuel in the fuel tank can be suppressed to 10% or less. For example, this temperature is set to about 40xc2x0 C.
With this configuration, in the case that the fuel temperature in the fuel tank is higher than the predetermined temperature, the opening operation of the control valve for opening and closing the evaporative fuel passage is disabled. Accordingly, when the fuel temperature rises, the closed condition of the fuel tank is maintained to thereby prevent changes in fuel component in the fuel tank and vacuum boiling in the fuel pump, so that the air-fuel ratio can be accurately controlled and smooth supply of the fuel can be ensured.
Preferably, the predetermined temperature is set as a lowermost fuel temperature at which vacuum boiling of the fuel tends to occur in a fuel pump for pumping up the fuel from the fuel tank when opening the control valve.
Preferably, the opening operation of the control valve is disabled for a predetermined period immediately after starting of the engine and in the case that the temperature of the engine is lower than a predetermined temperature.
Preferably, the evaporative emission control system further includes tank pressure detecting means for detecting the pressure in the fuel tank, and intake pressure detecting means for detecting the pressure in the intake system, wherein the disabling means disables the opening operation of the control valve in the case that the pressure in the fuel tank is lower than a pressure value obtained by adding the pressure in the intake system and a predetermined pressure.
Preferably the predetermined pressure is set to a value slightly larger than a maximum value of possible changes in the pressure in the intake system during the period between successive detections of the pressure in the fuel tank.
Alternatively, the predetermined pressure is set to a value slightly larger than a maximum value of pressure differences between an actual intake pressure and the detected intake pressure due to a detection delay in the intake pressure detecting means.
Other objects and features of the invention will be more fully understood from the following detailed description and appended claims when taken with the accompanying drawings.